Coal can't dispatch fast enough, and most coal plants are at the end of their lifecycle and need to be replaced. So you have to build new baseload capacity, usually gas. (And really, you still need a bunch of batteries to handle short-term fluctuations in renewables output.) The problem, then, is that you have all new coal or gas plants, which are expensive capital assets, that you're basically using for standby power. (And they incur operating and maintenance costs even when not running.) You can't say that electricity from renewables is cheap without accounting for the cost of that standby capacity renewables require. A nuclear plant, by contrast, doesn't need a bunch of coal or gas plants sitting around on standby. You can shut down that old coal plant, stop paying the workers, reuse the land, etc.